( 241 j 
S.G. before S.G 
Name. : : 
En heating {after heating. 
En ; 5152 hay 
Sillimanite Sonne EEE 
| 
CY Vl — 2 OA 
Disthene 3 a ane 
; 345 3.14 
Andalusite dee ep 
To determine the specifie gravity, I made, as much as possible, 
use of the floating method. The fluids I used were methyl-iodide 
(spec. gr. 351) and acetylenetetrabromide (spec. grav. 2.84). The 
instrument I used to fix these spee. grav. was the “Westphal-balance’’, 
except when the fluids were too light, in which case I made use of 
a xylolareometer. 
Consequently the results, as shown above, are pretty much the 
same, as those of VERNADSKY. 
The extinction of disthene, after the heating, had become parallel 
also, however before the melting temperature of copper (1100 degr. 
C.) had been reached. 
What is a strong argument against the change of disthene into 
sillimanite is its index, which I fixed in a way, indicated by Prof. 
SCHROEDER VAN DER Kork, i.e. by using fluids of which the refractive 
index is known). They were: methyl-iodide (#2 = 1.74); monobro- 
mine-naphtaline (2 == 1.66); monochlorine-naphtaline (2 = 1.64); 
mono-iodine-benzol (n = 1.62) and mixtures of them. The index of 
those fluids, I have fixed by using a Pulfrich with changeable 
refracting angle. 
Thus I could fix the index of very small pieces and moreover 
acquire a precision up to the second decimal. 
I have fixed the index only in the direction of the c-axis. The, 
double refraction not being great, this was sufficient. That index is 
the greatest, since in sillimanite, the ellipse of intersection, with the 
indicatrix, has its long axis in the direction of the c-axis. 
Before the heating process, the index of sillimanite was 1.68. 
Heating did not in the least affect it. 
Andalusite remains equally unaffected by it, it has an index of 1.64. 
1) See „Tabellen zur Mikroskopischen Bestimmung der mineralien nach ihrem 
Brechungsindex” by Dr, J. L, C, SCHROEDER VAN DER Kork. 
